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Abstract:

The present invention provides a molten sodium secondary cell. In some
cases, the secondary cell includes a sodium metal negative electrode, a
positive electrode compartment that includes a positive electrode
disposed in a liquid positive electrode solution, and a sodium ion
conductive electrolyte membrane that separates the negative electrode
from the positive electrode solution. In such cases, the electrolyte
membrane can comprise any suitable material, including, without
limitation, a NaSICON-type membrane. Furthermore, in such cases, the
liquid positive electrode solution can comprise any suitable positive
electrode solution, including, but not limited to, an aqueous sodium
hydroxide solution. Generally, when the cell functions, the sodium
negative electrode is molten and in contact with the electrolyte
membrane. Additionally, the cell is functional at an operating
temperature between about 100° C. and about 170° C. Indeed,
in some instances, the molten sodium secondary cell is functional between
about 110° C. and about 130° C.

Claims:

1. A molten sodium secondary cell, comprising: a sodium metal negative
electrode, which electrochemically oxidizes to release sodium ions during
discharge and electrochemically reduces sodium ions to sodium metal
during recharging; a positive electrode compartment comprising a positive
electrode disposed in a liquid positive electrode solution; and a sodium
ion conductive electrolyte membrane that separates the sodium metal
negative electrode from the liquid positive electrode solution, wherein
the sodium metal negative electrode is molten and in contact with the
conductive electrolyte membrane as the cell operates, and wherein the
cell functions at an operating temperature between about 100.degree. C.
and about 170.degree. C.

11. A molten sodium rechargeable cell, comprising: a sodium metal
negative electrode, which electrochemically oxidizes to release sodium
ions during discharge and electrochemically reduces sodium ions to sodium
metal during recharging; a positive electrode compartment comprising a
positive electrode disposed in an aqueous liquid positive electrode
solution; and a NaSICON-type, sodium ion conductive electrolyte membrane
that separates the sodium metal negative electrode from the liquid
positive electrode solution, wherein the sodium metal negative electrode
is molten and in contact with the conductive electrolyte membrane as the
cell operates, and wherein the cell functions at an operating temperature
between about 100.degree. and about 150.degree. C.

12. The cell of claim 11, wherein the liquid positive electrode solution
comprises between about 4% and about 50% sodium hydroxide, by weight.

13. The cell of claim 11, wherein the cell functions when the operating
temperature is between about 110.degree. C. and about 130.degree. C.

15. The cell of claim 11, wherein the liquid positive electrode solution
comprises from about 0 to about 50% sodium hydroxide, by weight; from
about 0 to about 96% glycerol, by weight; from about 0 to about 45%
borax, by weight; and from about 0 to about 93% water, by weight.

16. A method for providing electrical potential from a molten sodium
secondary cell, the method comprising: providing a molten sodium
secondary cell, comprising: a sodium metal negative electrode, which
electrochemically oxidizes to release sodium ions during discharge and
electrochemically reduces sodium ions to sodium metal during recharging;
a positive electrode system comprising a positive electrode disposed in a
liquid positive electrode solution; and a sodium ion conductive
electrolyte membrane that separates the sodium metal negative electrode
from the liquid positive electrode solution; and heating the sodium metal
negative electrode to a temperature between about 100.degree. C. and
about 170.degree. C. so that the sodium metal negative electrode is
molten and in contact with the sodium ion conductive electrolyte membrane
and so that the sodium metal negative electrode oxidizes to release the
sodium ions and allows the cell to discharge electricity.

18. The method of claim 16, wherein the liquid positive electrode
solution comprises between about 4% and about 50% sodium hydroxide, by
weight.

19. The method of claim 16, further comprising maintaining the
temperature of the sodium metal negative electrode between about
110.degree. and about 130.degree. C.

20. The method of claim 16, further comprising recharging the cell by
passing an electrical potential between the sodium metal negative
electrode and the positive electrode to cause the sodium negative
electrode to electrochemically reduce sodium ions to sodium metal.

21. The method of claim 16, wherein the liquid positive electrode
solution comprises from about 0 to about 50% sodium hydroxide, by weight;
from about 0 to about 96% glycerol, by weight; from about 0 to about 45%
borax, by weight; and from about 0 to about 93% water, by weight.

22. A molten sodium secondary cell, comprising: a negative electrode,
which electrochemically oxidizes to release sodium ions during discharge
and electrochemically reduces sodium ions to sodium metal during
recharging; a positive electrode compartment comprising a positive
electrode disposed in a liquid positive electrode solution; and a sodium
ion conductive electrolyte membrane that separates the negative electrode
from the liquid positive electrode solution, wherein the negative
electrode is molten and in contact with the conductive electrolyte
membrane as the cell operates, and wherein the cell functions at an
operating temperature between about 100.degree. C. and about 170.degree.
C.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Patent
Application No. 61/410,812, filed Nov. 5, 2010, which application is
incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates in general to batteries. More
particularly, the present invention provides a molten sodium-based
secondary cell (or rechargeable battery) with a sodium ion conductive
electrolyte membrane that operates at a temperature between about
100° Celsius ("C") and about 170° C.

BACKGROUND OF THE INVENTION

[0003] Batteries are known devices that are used to store and release
electrical energy for a variety of uses. In order to produce electrical
energy, batteries typically convert chemical energy directly into
electrical energy. Generally, a single battery includes one or more
galvanic cells, wherein each of the cells is made of two half-cells that
are electrically isolated except through an external circuit. During
discharge, electrochemical reduction occurs at the cell's positive
electrode, while electrochemical oxidation occurs at the cell's negative
electrode. While the positive electrode and the negative electrode in the
cell do not physically touch each other, they are generally chemically
connected by at least one (or more) ionically conductive and electrically
insulative electrolyte(s), which can either be in a solid or a liquid
state, or in combination. When an external circuit, or a load, is
connected to a terminal that is connected to the negative electrode and
to a terminal that is connected to the positive electrode, the battery
drives electrons through the external circuit, while ions migrate through
the electrolyte.

[0004] Batteries can be classified in a variety of manners. For example,
batteries that are completely discharged only once are often referred to
as primary batteries or primary cells. In contrast, batteries that can be
discharged and recharged more than once are often referred to as
secondary batteries or secondary cells. The ability of a cell or battery
to be charged and discharged multiple times depends on the Faradaic
efficiency of each charge and discharge cycle.

[0005] While rechargeable batteries based on sodium can comprise a variety
of materials and designs, most, if not all, sodium batteries requiring a
high Faradaic efficiency employ a solid primary electrolyte separator,
such as a solid ceramic primary electrolyte membrane. The principal
advantage of using a solid ceramic primary electrolyte membrane is that
the Faradaic efficiency of the resulting cell approaches 100%. Indeed, in
almost all other cell designs electrode solutions in the cell are able to
intermix over time and, thereby, cause a drop in Faradaic efficiency and
loss of battery capacity.

[0006] The primary electrolyte separators used in sodium batteries that
require a high Faradaic efficiency often consist of ionically conducting
polymers, porous materials infiltrated with ionically conducting liquids
or gels, or dense ceramics. In this regard, most, if not all,
rechargeable sodium batteries that are presently available for commercial
applications comprise a molten sodium metal negative electrode, a sodium
β''-alumina ceramic electrolyte separator, and a molten positive
electrode, which may include a composite of molten sulfur and carbon
(called a sodium/sulfur cell), or molten NiCl2, NaCl, and
NaAlCl4 (called a ZEBRA cell). Because these conventional high
temperature sodium-based rechargeable batteries have relatively high
specific energy densities and only modest power densities, such
rechargeable batteries are typically used in certain specialized
applications that require high specific energy densities where high power
densities are typically not encountered, such as in stationary storage
and uninterruptable power supplies.

[0007] Despite the beneficial characteristics associated with some
conventional sodium-based rechargeable batteries, such batteries may have
significant shortcomings. In one example, because the sodium
β''-alumina ceramic electrolyte separator is typically more
conductive and is better wetted by molten sodium at a temperature in
excess of about 270° C. and/or because the molten positive
electrode typically requires relatively high temperatures (e.g.,
temperatures above about 170° or 180° C.) to remain molten,
many conventional sodium-based rechargeable batteries operate at
temperatures higher than about 270° C. and are subject to
significant thermal management problems and thermal sealing issues. For
example, some sodium-based rechargeable batteries may have difficulty
dissipating heat from the batteries or maintaining the negative electrode
and the positive electrode at the relatively high operating temperatures.
In another example, the relatively high operating temperatures of some
sodium-based batteries can create significant safety issues. In still
another example, the relatively high operating temperatures of some
sodium-based batteries require their components to be resistant to, and
operable at, such high temperatures. Accordingly, such components can be
relatively expensive. In yet another example, because it may require a
relatively large amount of energy to heat some conventional sodium-based
batteries to the relatively high operating temperatures, such batteries
can be expensive to operate and energy inefficient.

[0008] Thus, while molten sodium-based rechargeable batteries are
available, challenges with such batteries also exist, including those
previously mentioned. Accordingly, it would be an improvement in the art
to augment or even replace certain conventional molten sodium-based
rechargeable batteries with other molten sodium-based rechargeable
batteries.

BRIEF SUMMARY OF THE INVENTION

[0009] The present invention provides a molten sodium secondary cell (or
rechargeable battery) that functions at a temperature between about
100° C. and about 170° C. While the described molten sodium
secondary cell can include any suitable component, in some non-limiting
implementations, the cell includes a sodium metal negative electrode, a
positive electrode compartment that includes a positive electrode
disposed in a liquid positive electrode solution, and a sodium ion
conductive electrolyte membrane that physically separates the negative
electrode from the positive electrode solution.

[0010] Generally, the sodium negative electrode comprises an amount of
sodium metal. In this regard, as the cell operates, the sodium negative
electrode is in a liquid or molten state. While the sodium negative
electrode may comprise any suitable type of sodium, including without
limitation, a pure sample of sodium or a sodium alloy, in some
non-limiting implementations, the negative electrode comprises a sodium
sample that is substantially pure.

[0011] The positive electrode in the positive electrode compartment can
comprise any suitable material that allows the cell to function as
intended. Indeed, in some non-limiting implementations, the positive
electrode comprises a wire, felt, mesh, plate, tube, foam, or other
suitable positive electrode configuration. In one non-limiting
embodiment, the positive electrode includes nickel, nickel oxyhydroxide
(NiOOH), nickel hydroxide (Ni(OH)2), sulfur composites, sulfur
halides, including sulfuric chloride, and/or any other suitable positive
electrode material.

[0013] The sodium ion conductive electrolyte membrane can comprise any
membrane (which is used herein to refer to any suitable type of
separator) that: selectively transports sodium ions, that is stable at
the cell's operating temperature, that is stable when in contact with
molten sodium and the positive electrode solution, and that otherwise
allows the cell to function as intended. Indeed, in some non-limiting
implementations, the electrolyte membrane comprises a NaSICON-type
membrane that is substantially impermeable to water. Accordingly, in such
implementations, the water impermeable electrolyte membrane can allow the
positive electrode solution to comprise an aqueous solution, which would
react violently if it were to contact the sodium negative electrode.

[0014] Where the electrolyte membrane comprises a NaSICON-type membrane,
the membrane can comprise any suitable kind of NaSICON-type membrane,
including, without limitation, a composite NaSICON membrane. In this
regard, and by way of non-limiting illustration, the membrane can
comprise any known or novel composite NaSICON membrane that includes a
dense NaSICON layer and a porous NaSICON layer, or a dense NaSICON layer
with a cermet layer, such as a NiO/NaSICON cermet layer.

[0015] The described secondary cell may operate at any suitable operating
temperature. Indeed, in some non-limiting implementations, the cell
functions (e.g., is discharged or recharged) while the temperature of the
cell is as high as a temperature selected from about 98° C., about
110° C., 120° C., about 130° C., about 150°
C., and about 170° C. Indeed, in some non-limiting
implementations, as the cell functions, the temperature of the negative
electrode is about 120° C.±about 10° C. In some
embodiments, the cell is pressurized ranging from about 1 psi to about 30
psi. In one embodiment, the cell may be pressurized in a range of about
10 psi to about 15 psi. These features and advantages of the present
invention will become more fully apparent from the following description
and appended claims, or may be learned by the practice of the invention
as set forth hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS

[0016] In order that the manner in which the above-recited and other
features and advantages of the invention are obtained and will be readily
understood, a more particular description of the invention briefly
described above will be rendered by reference to specific embodiments
thereof that are illustrated in the appended drawings. Understanding that
the drawings are not made to scale, depict only some representative
embodiments of the invention, and are not therefore to be considered to
be limiting of its scope, the invention will be described and explained
with additional specificity and detail through the use of the
accompanying drawings in which:

[0017] FIG. 1 depicts a schematic diagram of a representative embodiment
of a molten sodium secondary cell, wherein the cell is in the process of
being discharged; and

[0018] FIG. 2 depicts a schematic diagram of a representative embodiment
of the molten sodium secondary cell, wherein the cell is in the process
of being recharged;

[0019] FIG. 3 depicts a computer generated graph showing experimental
results showing the measured electrical potential over a period of time
for a representative embodiment of the molten sodium secondary cell;

[0020] FIGS. 4 and 5 each depict a computer generated graph showing
experimental results showing the measured voltage over an extended period
of time of a representative embodiment of the cell comprising molten
sodium on both sides of the NaSICON membrane;

[0024] FIG. 9 depicts a comparison of charge and discharge cycles for a
sodium/NiOOH cell having a NaSICON electrolyte separator membrane
operated at 120° C. with a nickel metal hydride cell having a
NaSICON electrolyte separator membrane operated at room temperature.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Reference throughout this specification to "one embodiment," "an
embodiment," or similar language means that a particular feature,
structure, or characteristic described in connection with the embodiment
is included in at least one embodiment of the present invention. Thus,
appearances of the phrases "in one embodiment," "in an embodiment," and
similar language throughout this specification may, but do not
necessarily, all refer to the same embodiment. Additionally, while the
following description refers to several embodiments and examples of the
various components and aspects of the described invention, all of the
described embodiments and examples are to be considered, in all respects,
as illustrative only and not as being limiting in any manner.

[0026] Furthermore, the described features, structures, or characteristics
of the invention may be combined in any suitable manner in one or more
embodiments. In the following description, numerous specific details are
provided, such as examples of suitable sodium negative electrodes,
positive electrode materials, liquid positive electrode solutions, sodium
ion conductive electrolyte membrane, etc., to provide a thorough
understanding of embodiments of the invention. One having ordinary skill
in the relevant art will recognize, however, that the invention may be
practiced without one or more of the specific details, or with other
methods, components, materials, and so forth. In other instances,
well-known structures, materials, or operations are not shown or
described in detail to avoid obscuring aspects of the invention.

[0027] As stated above, secondary cells can be discharged and recharged
and this specification describes cell arrangements and methods for both
states. Although the term "recharging" in its various forms implies a
second charging, one of skill in the art will understand that discussions
regarding recharging would be valid for, and applicable to, the first or
initial charge, and vice versa. Thus, for the purposes of this
specification, the terms "recharge," "recharged" and "rechargeable" shall
be interchangeable with the terms "charge," "charged" and "chargeable"
respectively.

[0028] The present invention provides a molten sodium secondary cell that
functions at an operating temperature between about 100° C. and
about 170° C. While the described cell can comprise any suitable
component, FIG. 1 shows a representative embodiment in which the molten
sodium secondary cell 10 comprises an negative electrode compartment 15
that includes a sodium metal negative electrode 20, a positive electrode
compartment 25 that comprises a positive electrode 30 that is disposed in
a liquid positive electrode solution 35, a sodium ion conductive
electrolyte membrane 40 that separates the negative electrode from the
positive electrode solution, a first terminal 45, and a second terminal
50. To provide a better understanding of the described cell 10, a brief
description of how the cell functions is provided below. Following this
discussion, each of the cell's components shown in FIG. 1 is discussed in
more detail.

[0029] Turning now to the manner in which the molten sodium secondary cell
10 functions, the cell can function in virtually any suitable manner. In
one example, FIG. 1 illustrates that as the cell 10 is discharged and
electrons (e.sup.-) flow from the negative electrode 20 (e.g., via the
first terminal 45), sodium is oxidized from the negative electrode 20 to
form sodium ions (Na+). FIG. 1 shows that these sodium ions are
respectively transported from the sodium negative electrode 20, through
the sodium ion conductive membrane 40, and to the positive electrode
solution 35.

[0030] In a contrasting example, FIG. 2 shows that as the secondary cell
10 is recharged and electrons (e.sup.-) flow into the sodium negative
electrode 20 from an external power source (not shown), such as a
recharger, the chemical reactions that occurred when the cell 10 was
discharged (as shown in FIG. 1) are reversed. Specifically, FIG. 2 shows
that as the cell 10 is recharged, sodium ions (Na+) are respectively
transported from the positive electrode solution 35, through the
electrolyte membrane 40, and to the negative electrode 20, where the
sodium ions are reduced to form sodium metal (Na).

[0031] Referring now to the various components of the cell 10, the cell,
as mentioned above, can comprise an negative electrode compartment 15 and
a positive electrode compartment 25. In this regard, the two compartments
can be any suitable shape and have any other suitable characteristic that
allows the cell 10 to function as intended. By way of example, the
negative electrode and the positive electrode compartments can be
tubular, rectangular, or be any other suitable shape. Furthermore, the
two compartments can have any suitable spatial relationship with respect
to each other. For instance, while FIG. 2 shows that the negative
electrode compartment 15 and the positive electrode compartment 25 can be
adjacent to each other, in other embodiments (not shown), one compartment
(e.g., the negative electrode compartment) is disposed, at least
partially, in the other compartment (e.g., the positive electrode
compartment), while the contents of the two compartments remain separated
by the electrolyte membrane 40 and any other compartmental walls.

[0032] With respect to the negative electrode 20, the cell 10 can comprise
any suitable sodium negative electrode 20 that allows the cell 10 to
function (e.g., be discharged and recharged) as intended. Some examples
of suitable sodium negative electrode materials include, but are not
limited to, a sodium sample that is substantially pure and a sodium alloy
comprising any other suitable sodium-containing negative electrode
material. In certain embodiments, however, the negative electrode
comprises or consists of an amount of sodium that is substantially pure.
In such embodiments, because the melting point of pure sodium is around
98° C., the sodium negative electrode will become molten above
that temperature.

[0033] With respect to the positive electrode 30, the cell 10 can comprise
any suitable positive electrode that allows the cell to be charged and
discharged as intended. For instance, the positive electrode can comprise
virtually any positive electrode material that has been successfully used
in a sodium-based rechargeable battery system. In some embodiments, the
positive electrode comprises a wire, felt, plate, tube, mesh, foam,
and/or other suitable positive electrode configuration. In one
non-limiting embodiment, the positive electrode comprises a nickel foam,
nickel hydroxide (Ni(OH)2), nickel oxyhydroxide (NiOOH), sulfur
composites, sulfur halides, including sulfuric chloride, and/or another
suitable material. Furthermore, these materials may coexist or exist in
combinations. For example a suitable positive electrode material may be
nickel oxyhydroxide (NiOOH) (e.g., when the cell is at least partially
charged) and nickel hydroxide (Ni(OH)2) (e.g., when the cell is at
least partially discharged). In certain embodiments, however, the
positive electrode comprises a nickel oxyhydroxide (NiOOH) electrode. It
is understood that a nickel oxyhydroxide electrode, even when fully
charged, will contain some amount of nickel hydroxide.

[0034] In some non-limiting embodiments where the positive electrode 30
comprises a nickel oxyhydroxide (NiOOH) electrode, the negative electrode
20 comprises sodium, and the positive electrode solution 35 (as discussed
below) comprises an aqueous solution, the reactions that occur at the
negative electrode and at the positive electrode and the overall reaction
as the cell 10 is discharged may occur as illustrated below:

Negative electrode NaNa++1e.sup.- (-2.71V)

Positive electrode NiOOH+H2ONi(OH)2+OH.sup.- (0.52V)

Overall Na+NiOOH+H2ONi(OH)2+NaOH (3.23V)

[0035] Accordingly, some embodiments of the describe cell 10, at least
theoretically, are capable of producing about 3.2V±0.5V at standard
temperature and pressure.

[0036] Moreover, some examples of overall reactions that may occur during
the discharging and charging of a cell in which the positive electrode 30
comprises a nickel oxyhydroxide (NiOOH) electrode, the negative electrode
20 comprises sodium, and the positive electrode solution 35 (as discussed
below) comprises an aqueous solution, are shown below:

(Discharge) NiOOH+H2O+Na+→Ni(OH)2+NaOH

(Charge) Ni(OH)2+NaOH→NiOOH+H2O+Na

[0037] With respect now to the positive electrode solution 35, the
positive electrode solution can comprise any suitable sodium ion
conductive material that allows the cell 10 to function as intended.
Additionally, in some embodiments, the positive electrode solution has a
higher sodium ion conductivity than does the electrolyte membrane 40
(described below). In one embodiment, the positive electrode solution
conductivity ranges between about 25 mS/cm and 500 mS/cm. In other
embodiments, the range may be between about 100 mS/cm and 300 mS/cm. In
other embodiments, between about 150 mS/cm and 250 mS/cm. The NaSICON
conductivity may range between about 20 and about 60 mS/cm. The NaSICON
conductivity may range between about 30 and about 45 mS/cm. In one
embodiment, for example, the conductivity of NaSICON GY, a NaSICON
material made by Ceramatec, Inc., is about 56 mS/cm at 120° C.
Other NaSICON compositions at this same temperature might have different
conductivity. It will be appreciated by those of skill in the art that
conductivity will vary as a function of temperature and the type of
ceramic materials.

[0039] By way of illustration, in some embodiments, the positive electrode
solution 35 comprises one or more of the following solutions: sodium
hydroxide and water; sodium hydroxide, borax, and water; glycerol and
sodium hydroxide; glycerol, sodium hydroxide, and water; glycerol and
borax; and borax and water.

[0040] The various ingredients in the positive electrode solution can have
any suitable concentration that allows the cell 10 to function as
intended. For instance, in some embodiments, the liquid positive
electrode solution comprises from about 0 to about 85% (e.g., between
about 4% and about 50%) sodium hydroxide, by weight; from about 0 to
about 96% glycerol, by weight; from about 0 to about 45% borax, by
weight; and from about 0 to about 93% water, by weight. In one
embodiment, the positive electrode solution ranges from about 30% to
about 75 wt % sodium tetraborate in water. In another embodiment, the
positive electrode solution ranges from about 55% to about 65 wt % sodium
tetraborate in water. By way of non-limiting illustration, Table 1 (shown
below) provides some non-limiting examples of suitable positive electrode
solutions.

[0041] While the positive electrode solutions in Table 1 are shown to have
specific concentrations, in other embodiments, the concentrations of the
sodium hydroxide, borax, and/or glycerol in such solutions can each be
modified by ±10%, by weight, and the concentration of the water or
glycerol filler can be changed accordingly.

[0042] In some embodiments, the positive electrode solution 35 has a
boiling point that is higher than the cell's operating temperature
(discussed below). In such embodiments, the boiling point of positive
electrode solution can be adjusted in any suitable manner that allows the
cell to function properly with the positive electrode solution. In this
regard, one non-limiting method for increasing the boiling point of an
aqueous positive electrode solution comprises increasing the
concentration of sodium hydroxide in the positive electrode solution. In
this manner, an aqueous sodium hydroxide positive electrode solution can
be modified to allow the cell to function at higher temperatures (e.g.,
up to about 170° C.). Another way to increase the boiling point is
to increase the pressure of the cell. In some non-limiting embodiments,
the electrolytic cell is pressurized cell to a pressure in the range of 5
to 25 psi. Operating the cell at a pressure higher than 25 creates a risk
of cracking a planar ceramic disk. However, if the ceramic is configured
as a tube though, the pressure of the cell could be increased to 200 psi,
because tube cells generally have greater mechanical strength and
increased surface area exposed.

[0043] With regards now to the sodium ion conductive electrolyte membrane
40, the membrane can comprise any suitable material that selectively
transports sodium ions and permits the cell 10 to function with a
non-aqueous positive electrode solution or an aqueous positive electrode
solution. In some embodiments, the electrolyte membrane comprises a
NaSICON-type (sodium Super Ion CONductive) material. In such embodiments,
the NaSICON-type material may comprise any known or novel NaSICON-type
material that is suitable for use with the described cell 10. Some
suitable examples of NaSICON-type compositions include, but are not
limited to, Na3Zr2Si2PO12,
Na1+xSixZr2P3-xO12 (where x is selected from 1.6
to 2.4), Y-doped NaSICON
(Na1+x+yZr2-yYySixP3-xO12,
Na1+xZr2-yYySixP3-xO12-y (where x=2,
y=0.12), and Fe-doped NaSICON
(Na3Zr2/3Fe4/3P3O12). Indeed, in
certain embodiments, the NaSICON-type membrane comprises
Na3Si2Zr2PO12. In still other embodiments, the
NaSICON-type membrane comprises known or novel composite,
cermet-supported NaSICON membrane. In such embodiments, the composite
NaSICON membrane can comprise any suitable component, including, without
limitation, a porous NaSICON-cermet layer that comprises NiO/NaSICON or
any other suitable cermet layer, and a dense NaSICON layer. In yet other
embodiments, the NaSICON membrane comprises a monoclinic ceramic.

[0044] Where the cell's electrolyte membrane 40 comprises a NaSICON-type
material, the NaSICON-type material may provide the cell 10 with several
beneficial characteristics. In one example, because NaSICON-type
materials, as opposed to a sodium β''-alumina ceramic electrolyte
separator, are substantially impermeable to, and stable in the presence
of, water, NaSICON-type materials can allow the cell to include a
positive electrode solution, such as an aqueous positive electrode
solution, that would otherwise be incompatible with the sodium negative
electrode 20. Thus, the use of a NaSICON-type membrane as the electrolyte
membrane can allow the cell to have a wide range of battery chemistries.
As another example of a beneficial characteristic that can be associated
with NaSICON-type membranes, because such membranes selectively transport
sodium ions but do not allow the negative electrode 20 and the positive
electrode solutions 35 to mix, such membranes can help the cell to have
minimal capacity fade and to have a relatively stable shelf life at
ambient temperatures.

[0045] With reference now to the terminals 45 and 50, the cell 10 can
comprise any suitable terminals that are capable of electrically
connecting the cell with an external circuit, including without
limitation, to one or more cells. In this regard, the terminals can
comprise any suitable material and any suitable shape of any suitable
size.

[0046] In addition to the aforementioned components, the cell 10 can
optionally comprise any other suitable component. By way of non-limiting
illustration FIG. 2 shows an embodiment in which the cell 10 comprises a
heat management system 55. In such embodiments, the cell can comprise any
suitable type of heat management system that is capable of maintaining
the cell within a suitable operating temperature range. Some examples of
such heat management systems include, but are not limited to, a heater,
one or more temperature sensors, and appropriate temperature control
circuitry.

[0047] The described cell 10 may function at any suitable operating
temperature. In other words, as the cell is discharged and/or recharged,
the sodium negative electrode may have any suitable temperature. Indeed,
in some embodiments, the cell functions at an operating temperature that
is as high as a temperature selected from about 120° C., about
130° C., about 150° C., and about 170° C. Moreover,
in such embodiments, as the cell functions, the temperature of the
negative electrode can be as low as a temperature selected from about
120° C., about 115° C., about 110° C., and about
100° C. Indeed, in some embodiments, as the cell functions, the
temperature of the negative electrode between about 100° C. and
about 150° C. In other embodiments, the cell functions at a
temperature between about 100° C. and about 130° C. In yet
other embodiments, however, as the cell functions, the temperature of the
negative electrode is about 120° C.±about 10° C.

[0048] In addition to the aforementioned benefits of the cell 10, the
described cell may have several other beneficial characteristics. By way
of example, by being able to operate in a temperature range between about
100° and about 150° C., the cell 10 may operate in a
temperature range that is significantly lower the operating temperature
of certain conventional molten sodium rechargeable batteries.
Accordingly, the described cell may require less energy to heat and/or
dissipate heat from the cell as the cell functions, may be less dangerous
use or handle, and may be more environmentally friendly.

[0049] The following examples are given to illustrate various embodiments
within, and aspects of, the scope of the present invention. These are
given by way of example only, and it is understood that the following
examples are not comprehensive or exhaustive of the many types of
embodiments of the present invention that can be prepared in accordance
with the present invention.

EXAMPLE 1

[0050] In one example, an embodiment of the described cell 10 was prepared
to contain a sodium negative electrode 20 and a positive electrode
solution 35 comprising an aqueous solution that included sodium hydroxide
at a concentration of about 50%, by weight. The cell was then heated so
that the sodium negative electrode became molten. As the cell was
operated at an operating temperature of about 120° C., the
electrical potential (in volts) of the cell was measured for around 25
hours. The results of this test are illustrated in FIG. 3. Specifically,
FIG. 3 shows that as the described cell operated, the cell was capable of
providing up to almost 1.6V for a prolonged period of time. The open
circuit voltage of this cell was measured to be 1.75V; however the
positive electrode material used was not a NiOOH material, which is why
the open circuit voltage was significantly lower than the theoretical
voltage of 3.23V. The full capacity (in mAhr) was not recorded for this
demonstration cell. The discharge current for the final cycle was
increased to 1 mA/cm2.

EXAMPLE 2

[0051] In another example, in order to show that a NaSICON-type
electrolyte membrane is stable in the described cell 10 with a molten
sodium negative electrode 20, an embodiment of the cell was prepared with
a sodium negative electrode, a NaSICON-type membrane (namely membrane
comprising Na3Zr2Si2PO12), and molten sodium. The
cell was then operated for about 1650 hours at an operating temperature
of about 112° C. and with a controlled current density of about 50
mA/cm2. Furthermore, during operation time, the NaSICON was cycled
approximately 200 times. This demonstrates the feasibility of using
NaSICON in the presence of molten sodium, something never before thought
possible.

[0052] The experimental results from this second example are shown in
FIGS. 4 and 5. Specifically, FIG. 4 illustrates the measured voltage of
the experimental embodiment of the cell over the 1650 hours of operation
time. In this regard, FIG. 4 shows that after the first 200 hours, or so,
the interfacial resistance at the molten Na/NaSICON interface improved
and the cell's voltage was reduced (interfacial resistance can include
improved wetting characteristics). Furthermore, FIG. 5, which shows the
final 100 hours of the cell's operation, shows that for the final 100
hours of the cell's operation, the cell's voltage cycles remained
substantially uniform. Accordingly, this second example shows that in a
molten sodium secondary cell comprising a NaSICON-type membrane, the
described cell is functional and the NaSICON-type membrane can be
relatively stable in the presence molten sodium for a prolonged period of
time.

EXAMPLE 3

[0053] In this example, an embodiment of the described cell 10 was
prepared to contain a sodium negative electrode 20 and a NiOOH positive
electrode 30. A NaSICON membrane approximately 1 mm thick and having a
diameter of about 1 inch separated the negative and positive electrode
compartments. The positive electrode solution 35 comprised an aqueous
solution that included borax at a concentration of about 30%, by weight.
The cell was then heated so that the sodium negative electrode became
molten. The cell was allowed to come to temperature for approximately one
hour before being charged. The cell was charged at a constant 15 mA
current. As the cell was operated at an operating temperature of about
120° C. and an external pressure of about 16 psi.

[0054] FIG. 6 shows the initial charging curve at 15 mA for four hours at
120° C. The cell was cycled under a variety of test procedures to
gain a better understanding of how the NiOOH electrode performs at the
elevated operating temperature because no known published literature
exists for NiOOH electrodes at molten sodium operating temperatures.

[0055] The initial charging was done at a constant current. This method
limits the number of cycles that can be done in a given time period, and
it is not necessarily the best for the nickel electrode. There are two
factors to be considered when creating a charge regime. The first is that
a nickel electrode is at its highest resistance at the beginning of
charge, when the majority of the material is in the form of Ni(OH)2,
and that the electrode is most likely to evolve oxygen at the end of
charge. To accommodate both these things, a step charge may be desirable.
The second factor is the charge (and step) termination. There are
multiple ways to determine the end of charge. These include end of charge
voltage, capacity limits (coulomb counting), -dV/dt, temperature and
pressure limits.

[0056]FIG. 7 demonstrates the slight variation in charge length on the
charging curves from Cycle 6 and 7. They are identical in structure, but
not in duration. Cycles 6 and 7 demonstrate that for a slightly more than
30% increase in charge, there is only a 15% increase in discharge
capacity. It has been widely established that nickel hydroxide electrodes
have poor charge acceptance at higher temperatures. It is likely that the
results shown in FIG. 7 reflect that trend. There are some options that
could increase the utilization of the material, including a high
temperature formulation, and increased compression on the nickel
electrode. These modifications should increase the gap between mid-point
charge voltage and the oxygen evolution potential, which has been linked
to improved charge acceptance.

[0057] There is a significant increase in resistance as the cell cycles.
There are multiple reasons that the resistance of a nickel oxyhydroxide
electrode might increase over time. In this case, two reasons seem the
most likely. One possibility is that there is a physical degradation of
the nickel electrode occurring during cycling. Nickel electrodes, such as
those used in conventional in NiMH batteries, do not normally cycle at
high heat, without compression, or with excess electrolyte.

[0058] FIG. 8 shows the discharge curves of Cycles 6 and 7. There is
capacity increase with each cycle, as well as a dramatic increase in cell
resistance.

[0059] Table 2 shows the cycle life of the Na/NiOOH cell with NaSICON
electrolyte membrane used in this example. The discharge cycles never
exceeded 20 mAh, which is presumably a C/2.25 rate. The cell's primary
purpose was to create the baseline programming for future testing. As
such, it was put on a 10mA discharge test for Cycles 14-16. This cell was
unable to discharge at that rate, and so it was considered unusable. When
the cell was disassembled, there was no visible leak path outside of
either half or between the two halves of the cell assembly. However, when
the sodium half was disassembled, there had clearly been a reaction, as
that the majority had become a white powdery solid.

[0060] This example demonstrates the effectiveness of a low-temperature
molten sodium secondary cell using a sodium ion conductive electrolyte
membrane, such as a NaSICON-type material. In addition, the nickel
oxyhydroxide and nickel hydroxide nickel positive electrode works in the
high temperature setting of a molten sodium cell.

EXAMPLE 4

[0061] The performance of the Na/NiOOH cell, as described above in Example
3, operated at 120° C. was compared to a nickel metal hydride
(NiMH) battery operated at room temperature. The NiMH battery was
constructed using a metal hydride negative electrode from a commercial,
unused metal hydride battery. A NaSICON electrolyte membrane was used
having the same composition, size, and thickness as used in the cell of
Example 3. The positive electrode was a freshly prepared Ni(OH)2
electrode was used having a similar theoretical capacity as the
Ni(OH)2 cathode of Example 3. A 35 wt. % NaOH solution was added to
both the negative and positive electrode compartments. The cell was
operated at room temperature. FIG. 9 depicts a comparison of charge and
discharge cycles for the Na/NiOOH cell of Example 3 operated at
120° C. with the NiMH cell operated at room temperature. The cells
were discharged at a rate of C/2, that it, a rate of one half the cell's
capacity.

[0062] This example demonstrates that the nickel oxyhydroxide and nickel
hydroxide nickel positive electrode works in the high temperature setting
of a molten sodium cell. As anticipated, the molten sodium cell operated
at higher power and lower capacity compared to the room temperature cell.
The higher power is due to the higher operating voltage. The lower
capacity is due to the inability to fully charge the cell because of
close oxygen evolution potential under charging conditions.

[0063] While specific embodiments and examples of the present invention
have been illustrated and described, numerous modifications come to mind
without significantly departing from the spirit of the invention, and the
scope of protection is only limited by the scope of the accompanying
claims.